The radiation continuum and the evolution of frog diversity

Most of life’s vast diversity of species and phenotypes is often attributed to adaptive radiation. Yet its contribution to species and phenotypic diversity of a major group has not been examined. Two key questions remain unresolved. First, what proportion of clades show macroevolutionary dynamics similar to adaptive radiations? Second, what proportion of overall species richness and phenotypic diversity do these adaptive-radiation-like clades contain? We address these questions with phylogenetic and morphological data for 1226 frog species across 43 families (which represent >99% of all species). Less than half of frog families resembled adaptive radiations (with rapid diversification and morphological evolution). Yet, these adaptive-radiation-like clades encompassed ~75% of both morphological and species diversity, despite rapid rates in other clades (e.g., non-adaptive radiations). Overall, we support the importance of adaptive-radiation-like evolution for explaining diversity patterns and provide a framework for characterizing macroevolutionary dynamics and diversity patterns in other groups.


Original variables
pPC-1 pPC-2 pPC-3 pPC-4 pPC-5 Head length 0.114 -0.089 -0.037 -0.377 0. We size-corrected and ln-transformed all variables prior to analysis.We also took the square-roots and cube-roots of the area and volume measurements, respectively, prior to size correction.Source Data can be found within Supplementary Code 1.

Measurement Description Snout-vent length
Length measured from tip of the snout to the cloaca while viewing the animal from the dorsal side Head length Length measured from tip of the snout to the posterior-most part of the mouth while viewing the animal dorso-laterally.

Head width
Length measured transversely, from either lateral side of the posterior-most position of the mouth to the other while viewing the animal from the dorsal side.

Upper front limb length
Length measured from the axilla, where the arm exits the body, to the tip of the elbow, while viewing the animal from the ventral side.Forearm length Length measured from the tip of the elbow to the base of the thumb on the manus, while viewing the animal from the ventral side Hand length Length measured from the proximal-most base of the thumb to the distal tip of the longest front limb digit, while viewing the animal from the ventral side Thigh length Length measured from the cloaca to the tip of the knee while viewing the animal from the ventral side Crus length Length measured from the tip of the knee to the proximal-most position of the tarsus, at the ankle.We identified the ankle as the point where more distal segments of the hind limb bend.We measured this while viewing the animal from the ventral side.

Tarsus length
Length measured from the ankle to the proximal-most edge of the inner metatarsal tubercle while viewing the animal from the ventral side Foot length Length measured from the proximal-most edge of the inner metatarsal tubercle to the tip of the longest hind limb digit while viewing the animal from the ventral side Front limb length The sum of the upper front limb, forearm, and hand lengths Hind limb length The sum of the thigh, crus, tarsus, and foot lengths Interdigital webbing area The sum of area measurements between hind limb digits from photographs of outstretched digits pressed against a glass slide Forelimb digit tip area The sum of area measurements of forelimb digit tips from photographs of digit tips lightly pressed against a glass slide Hindlimb Digit Tip Area The sum of area measurements of hindlimb digit tips from photographs of digit tips lightly pressed against a glass slide Inner metatarsal tubercle area Area measurement of the inner metatarsal tubercle from photographs of the foot gently pressed against a glass slide Thigh volume Volume approximated as two cones sharing an elliptical base, pointed away from one another and meeting at the mid-point of the thigh.Lengths of the major and minor axes of the ellipse are the depth and width of the thigh at the mid-point, measured perpendicular to one another, which measurement is considered major or minor is arbitrary.The area of an ellipse is given by A = All lengths in mm, areas in mm 2 , and volumes in mm 3 .
πab, where a and b are the semi-axes of the ellipse.The volume of a cone with an elliptical base is given by V = HA/3, where H is the height of the cone and A is the area of the ellipse.Since we approximate the volume of the entire thigh as two cones sharing the same base, we can sum their heights as the entire leg segment length (i.e., thigh) to calculate the volume of both cones together.

Crus volume
Similar to thigh volume but using crus measurements.